This invention relates generally to the field of turbines or power plants that produce electricity by harnessing the flow of water, whether the flow is uni-directional, such as found in a river or oceanic currents, or bi-directional, such as a tidal flow, and more particularly relates to such devices wherein the fluid flow causes rotation of a large propeller-type rotor having an annular outer rim disposed within a large annular housing. Even more particularly, the invention relates to such devices wherein the turbine is submerged within the body of water.
Production of electricity using hydroelectric turbines is well known. Typically, turbines are mounted in dams such that controlled fluid flow causes rotation of a propeller-type rotor or blades. Such relatively rapid water flow conditions are known as high head conditions. It is also known to place turbines in low head conditions, such as produced by tidal flow in a bay, at the mouth of a river or offshore.
While most turbines are constructed to have a central rotating shaft onto which the blades or runners are mounted, it is also known to produce open-centered turbines, also known as rim-mounted turbines. Turbines having open-centered rotors, where the blades are mounted between inner and outer annular rings or rims and where the energy is transferred through the outer rim to an annular housing that retains the rotor, can be particularly successful in low head conditions, i.e., in slower currents.
Examples of open center, rim-mounted turbines can be seen U.S. Pat. No. 5,592,816 issued Jan. 14, 1997, and reissued as RE38,336 on Dec. 2, 2003, U.S. Pat. No. 6,648,589 issued Nov. 18, 2003, U.S. Pat. No. 6,729,840 issued May 4, 2004, and U.S. Patent Appl. Publication US2005/0031442 published Feb. 10, 2005 (Ser. No. 10/633,865). Examples of hydroelectric turbines used in low head (tidal flow) conditions can be seen in U.S. Pat. No. 4,421,990 to Heuss et al., U.S. Pat. Nos. 6,168,373 and 6,406,251 to Vauthier, UK Patent Appl. No. GB 2,408,294 to Susman et al., and WIPO International Publication WO 03/025385 to Davis et al.
Fluid powered turbines are seen as environmentally safe replacements for electrical power plants that utilize fossil fuels or atomic energy. In the harnessing of wind or water to produce electricity on a large scale capable of powering industrial complexes, towns, cities, etc., it is necessary to provide large numbers of turbines, and it is necessary that the turbines be as large as practical in order to maximize the amount of electricity produced by each turbine. The rotor blades of these turbines are multiple meters in length, with some experimental designs having blades exceeding 50 meters in length.
As the length of the rotor blades is increased, structural and manufacturing challenges are presented that are not encountered in smaller turbines or generators. For shaft-mounted turbines, it is difficult to provide long blades that are both strong and light. The rim-mounted turbines provide a solution to this problem by providing annular support to each end of the blade, with the outer support rim being retained within a housing having an annular slot or channel. For generation of electrical power, a large number of magnets are spaced along the annular support rim and a large number of coils are spaced along the receiving channel in the stator housing. The magnetic field established by the rotor field system passes across the gap that separates the rotor and the stator. Rotation of the rotor causes the magnetic flux linkage with the coils to change, inducing an electro-magnetic force in the coils.
In rim-mounted turbines, the weight of the rotor is borne by the lower half of the housing since there is no central supporting shaft or axle. For large turbines this load and the resulting friction effects can be significant, both as to initial start-up of the rotor within the housing and in the overall efficiency of the turbine once rotation has been achieved. Increased rotor weight means increased resistance to rotation, meaning that greater fluid flow is required to overcome the inherent inertia and friction. This is a particular problem for hydroelectric turbines used in low head conditions.
It is an object of this invention to provide an improved structure for a hydroelectric turbine wherein the weight of the rotor is reduced such that the rotor is buoyant. It is a further object to provide such a turbine wherein the reduction in weight is accomplished by providing buoyancy chambers in the rotor, such that for turbines submerged in water the negative gravity effects resulting from the large weight of the rotor are reduced or countered by the increased buoyancy of the rotor.